I'm shopping for a new car battery for 1998 Subaru Outback (2.5l). For the physical dimensions the battery has to match, I see some variance in capacity (Ah) rating, and in price too, of course.

The intuition goes, as usual, "higher is better". The obvious advantage--if I replace current 55Ah battery with 60Ah-one, I'll be able to, say, play music with engine off for longer, and still be able to start the car. Are there disadvantages of higher capacity too, though?

Also, apart from capacity, which, if any, of the other characteristics (CA, CCA, HCA, RCM etc.) make significant difference in practice and should be paid attention to?

6 Answers
6

In most cases, the stock-size battery is correct, and that's what you should stick with.

A smaller battery is likely to fail you sooner, unless you live somewhere without a winter (Hilo?).

A larger battery is an extra expense, extra toxins, extra weight, and won't give you dramatically longer life.

CCA (cold cranking amps) is the main thing to pay attention to. This is a measure of how much current the battery can provide at 0 deg F, as @Pangea points out. Batteries work better when they are warm, so the colder it gets, the less current it can provide.

Starting an engine requires a lot of current, over a very short time. Meanwhile, cold engines are harder to start, requiring more power to get them going.

As batteries age, their internal resistance increases, reducing CCA. Total capacity decreases at the same time. This is why a newer battery can start your car in any weather, even if you leave the headlights on overnight, while an old battery might only start the car when fully charged, in good weather.

Starter batteries are optimized for short-duration, high-current draws, followed immediately by recharging. They are built with thinner plates with more surface area, which speeds up the chemical reactions that release the energy. If you draw them down to 1/2 charge, and then leave them like that for a while, hard crystals will form on the plates, which won't re-desolve easily on recharging. That reduces capacity and CCA over time. So, keep your starter battery fully charged, and avoid dropping below 80% during non-starter usage.

(Deep-cycle batteries, like on golf carts, fork lifts, some boats, some RVs are able to tolerate being discharged deeper and left that way longer than starter batteries, but they really don't like a high-current draw like for starting an engine. Best to draw on them gently, and never below 50% if you can help it.)

So, a regular car battery is not a good device for running appliances for a long time, without the engine running. However, a stock car stereo doesn't draw much current, so you can probably get away with it for a while without a problem. But if you leave it on all night, you may drain your starter battery enough to damage it.

Increasing your battery size will allow you to run those appliances for a little longer, but it's still not the right technology.

In an RV or boat, you have a starter battery and a separate "house" battery, which is deep-cycle. Then use a battery isolator switch to use only the starter to start, then recharge it, then recharge the house battery from the alternator. (You can also use the house battery to assist the starter when the starter battery is low.) Some people build these in to their cars if they have really big stereos, or a small camping setup with fridge in a van or pick-up camper. You can learn more from RV articles, like this one: http://www.phrannie.org/battery.html

Consider getting a LiFePO4 starter battery, for example from A123 cells. It will give much better performance in lower temperatures then lead-acid. It will be several times more expensive, but considering it will last several times more (probably outlive your car - but you can take it with you to your next car), and can be discharged to almost 100% (not totally 100%, don't go below 2V per cell), it can work-out cheaper then lead-acid in the long term.

Such battery will also be 4-5 times lighter and smaller for similar capacity and cranking power, has no acid and can be mounted in any orientation, and is very safe to charge, no hydrogen explosion risk.

As for cranking power, from such battery you can get about 50C for 10 seconds (50 times more current in A then its Ah capacity), and 30C continuously. From a lead-acid battery you can only get about 5C to 6C cranking power, so if you only care about cranking power, you need only a A123 battery with 5-10 times less capacity to get the same cranking power.

You can either buy one, which will be quite expensive, or construct one out of bare 18650 or 26650 cells from China - which is a lot of work, but cheaper.

Not a battery specialist, but I believe that CC amps (basically some sort of rated short-circuit current) would be important parameter regarding the engine type i.e. if you have a diesel or a uber tuned racer with large compression ratios or just a large engine (V8s, V12s), you will need large currents to start such an engine.

I live in Massachusetts and this past winter saw several negative degree mornings, during which it was very difficult for the engine to turn over. After replacing my battery with a higher CCA rating, it was much easier to get started in the cold mornings.

This aligns with my experience. For my car, manufacturer specifies battery with 490A CCA. The dying battery I'm about to replace has just 450A written on its side--and probably even less in reality as it's old and (ab-)used. In -20°C mornings it is barely able to start the engine. I'll make sure the new battery exceeds the 490A minimum.
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Pēteris CauneMar 15 '11 at 10:31

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I live in Alberta Canada, and I don't go with anything less than 800 Cold Cranks. If I do, I'll be stranded at -40.
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Chase FlorellMar 15 '11 at 16:21

I'm surprised 800 CCA is enough at -40. My little 4-cyl barely starts at -20 with an 800 CCA. I upgraded it to 1200 CCA (had to rig up new battery mounts too, since it was so much larger than stock). :-)
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Brian KnoblauchDec 12 '11 at 18:36

I wouldn't say there are disadvantages per se. My personal policy is that whenever a battery needs to be replaced on any car, I go with an Optima red or yellow top gel cell battery. They don't leak, the yellow ones are deep-discharge friendly so you can run them down (say by accidentally leaving your lights on) and not kill them, and they seemingly last forever. Simply put, last battery you'll ever buy. Sure they're $150-$200, but if I get 10 years of use out of one, I feel its worth it.

Thanks for advice. Checking prices--Optima Red costs $270 here in Latvia. Realistically I don't expect to own this car for more than 2-3 years, so I'm hesitant to invest that much in battery. OTOH though, if the battery outlives several cars, it could indeed pay off. Guess I'll do my homework on gel batteries!
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Pēteris CauneMar 14 '11 at 17:34

Yellow Tops also rock for car audio. Rewind your alternator for a little more amperage and let 'er rock. No need for a cap... just sayin ;)
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Chase FlorellMar 15 '11 at 16:20

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Optima Yellow Tops are awesome. I've got two of them, would have three if they came in the right size. I don't have any fancy stereo gear, I got them because they'll come back from truly ridiculous discharged states and work fine.
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Mark Johnson♦Dec 23 '11 at 17:53